1. Field of the Invention
This invention relates to telecommunication interface circuitry incorporating modulators and demodulators (modems) which operate according to frequency shift keyed (FSK) techniques. Communication of digital data over telephone lines requires the use of interface circuitry for converting serial digital data to an analog signal which is compatible with the characteristics of a telephone line or other communication channel. The interface generally includes a device known as a modem, which is operative to convert an asynchronous serial digital data stream into an analog signal compatible with the transmission characteristics of specific types of channels such as a voiceband channel and which receives an analog signal over such a channel and converts the analog signal into a digital data stream. Various communication standards have been established for communication over telephone lines. A typical form of communication is by frequency shift keyed (FSK) modulation in the data rate range of 300-1200 symbols per second. FSK modulation comprises transmission of one bit of information per symbol wherein the bit value is determined by the selection of one of two preselected frequencies.
Standards have been adopted in the United States and in Europe for asynchronous serial communication employing FSK modulation. These standards are incorporated into the modems known as the Bell 103/113 and Bell 202 in the United States and the CCITT V.21 and V.23 modems in Europe. In addition, standards have been established for interface control for asynchronous serial data communication. EIA Standard RS-232C defines essential terminal control signals required for communication over serial asynchronous lines. CCITT Standard V.24 specifies the essential terminal control signals for the European standard equipment. A relatively inexpensive single-chip programmable modem is likely to find wide application and acceptance in telecommunications. For example, a particular application is in terminals of remote access information systems such as the British VIEWDATA system, the French Postal Telephone and Telegraph electronic telephone directory, the Advanced Communication System of American Telephone and Telegraph Company, as well as in terminals used for remote diagnosis of computers and instruments.